1. Field of the Invention
The present invention generally relates to a method and apparatus for the simulation of the real-time error performance of digital data transmitted over radio links and, more particularly, to a system which allows the user to evaluate the performance of digital radio communications systems in real time in a laboratory thereby avoiding the need for expensive field measurements on prototype radio equipment.
2. Description of the Prior Art
Frequency-selective fading and flat fading are known to exist in portable radio communication channels. These propagation effects can cause bit errors in digital radio communications. When the bandwidth of the transmitted data stream is larger than the coherence bandwidth of the channel, frequency-selective fading will occur. Frequency-selective fading causes signal spreading and, as a result, introduces intersymbol interference (ISI) and bit errors in received signals. On the other hand, if the bandwidth of the data is much less than the bandwidth of the channel, flat fading will occur. Although the flat fading channel will not cause any signal spreading in the transmitted signal, the fading along with the added thermal and impulse noise in the channel can still cause bit errors, as can imperfect filters. For data transmission systems, the error bursts due to reception during signal nulls of a fading channel are a primary concern, and understanding the burstiness of the channel is necessary to implement successful antenna diversity and/or coding techniques. In addition, mobile and indoor channels are subject to rapid changes in signal strength and multipath conditions.
As demand grows for digital wireless communications systems, the accurate prediction of average and instantaneous bit error rates (BER) for different modulation schemes becomes increasingly important in system design. For example, the extent of coding required for U.S. digital cellular data products will depend on the quality of transmission sustained in mobile channels. BER predictions not only provide an understanding of the performance of the modulation method and coding requirements in the operating environment, but also reveals the limits of data rate and channel capacity.
Currently, communications companies and various government agencies have an urgent need to test proposed speech and video coding in digital modulation communication systems. The communications technologies range from the proposed new digital cellular standard to high definition television (HDTV). Presently, one needs to perform expensive field measurements on prototype radio equipment or use complicated hardware simulators in conjunction with prototype radio equipment to test the digital transmission quality of new services over radio links.
It is known in the prior art to use a statistical model for channel and noise levels in the operating environment. T. S. Rappaport, S. Y. Seidel and K. Takamizawa describe examples in "Statistical Channel Impulse Response Models for Factory and Open Plan Building Radio Communication System Design", IEEE. Trans. Commun., vol. COM-39, no. 5, May 1991, See also J. I. Smith, "A Computer Generated Multipath Fading Simulation for Mobile Radio", IEEE Trans. Veh. Techno., vol. VT-24, no. 3, Aug. 1975, pp. 39-40, and G. A. Arrendondo and W. H. Chriss, "A Multipath Fading Simulator for Mobile Radio", IEEE Trans. Commun. vol. COM-21, no. 11, Nov. 1973, pp. 1325-1328.
Existing simulators allow real-time generation of bit errors; however, these simulators do not simulate any radio systems. They only generate errors (or bursts of errors) at random. It is left to the user to determine the simulator settings to produce a very rough approximation of the radio transmission. The existing simulators are therefore very limited in their ability to simulate bursty errors that occur in a mobile channel.